This invention relates to the testing of electrical installations and in particular, to the testing of such installations which include residual current devices (RCDs).
To ensure the safety of an electrical installation a measurement is made of the impedance between the live or phase terminal and earth at various points: this test is commonly known as a `loop test`. Traditionally the loop test applies a load between live and earth such that a high current flows for a relatively short period of time (typically 25A according to International standards). The voltage drop due to the current is used to calculate the loop impedance.
Installations often include a residual current device (RCD) to detect a fault current flowing to earth by sensing an imbalance in the live and neutral currents flowing to the installation. An imbalance above a set level causes the RCD to `trip`, and disconnect the mains to the installation.
A loop test applied on the load side of an RCD applies a large current imbalance which trips the RCD. The loop test therefore cannot be completed and there may also be inconvenience to users as the mains supply fails.
EP-A-0295880(Robin Electronics) describes a method of testing which prevents an RCO from tripping while a standard loop rest is carried out. However newer RCD designs are susceptible to tripping even using this method.
Accordingly the present invention seeks to provide a method of performing a loop test without tripping RCDS.